1. Field of the Invention
The present invention generally relates to image rejection. More specifically, the present invention provides independent tuning of in-phase and quadrature-phase local oscillator signals for improved image rejection.
2. Background Art
A generalized in-phase/quadrature-phase transmitter uses two mixers to generate a transmitter output signal. A first mixer receives a first input signal and an in-phase local oscillator signal and produces a first output signal. A second mixer receives a second input signal and a quadrature-phase local oscillator signal and produces a second output signal. The second output signal is subsequently subtracted from the first output signal to form a transmitter output signal.
The transmitter output signal includes only a main signal when a phase difference between the in-phase local oscillator signal and the quadrature-phase local oscillator is exactly equal to 90°. The transmitter output signal, however, will also include an image signal when the phase difference between the in-phase local oscillator signal and the quadrature-phase local oscillator is not exactly equal to 90°. The image signal is an undesirable signal that can interfere with the subsequent detection of the main signal. A phase imbalance between the in-phase local oscillator signal and the quadrature-phase local oscillator signal can be reduced by monitoring the image signal and iteratively adjusting the phase of either the first or second input signals to account for the oscillator phase imbalance. Such techniques, however, may suffer from significant propagation delays and settling time delays associated with adjusting the phases of the first and second input signals. Further, current techniques for reducing a phase imbalance between the in-phase and quadrature-phase local oscillator signals do not provide independent tuning of the phases of the in-phase and quadrature-phase local oscillator signals.